1. Field of the Invention
The present invention relates to a phosphor, a method for manufacturing the same, and a light emitting diode. More particularly, the present invention relates to a phosphor having moisture and/or heat stability, a method for manufacturing the same, and a light emitting diode using the phosphor.
2. Description of the Related Art
Phosphors known in the art include oxide-based phosphors, sulfide-based phosphors, recently developed nitride-based phosphors, and the like. The phosphors are typically excited by light from a blue or UV light emitting diode chip, and require moisture or heat stability to maintain good light emitting characteristics of a light emitting diode.
Currently, blue light emitting diode chips have been gradually increased in size to achieve application of white light emitting diodes to common illumination. With this trend in the art, when driven under rated current without a specific device for heat dissipation, the light emitting diode chip undergoes high heat of approximately 120° C. or more, which is generated in a brief instance from a light emitting layer of the chip and causes significant reduction in luminous intensity of the phosphor. Generally, when the temperature increases around the phosphor, the phosphor has a widened spectrum resulting from interference between a host lattice and activators and lattice expansion caused by lattice vibration, and experiences reduction in the luminous intensity due to variation of chromaticity coordinate and weakening of a crystal field. Additionally, the oxide-based phosphors such as YAG:Ce and (Ba, Sr, Ca)2SiO4:Eu are likely to be affected by an increase in temperature of the light emitting diode chip and thus undergo a rapid deterioration of fluorescent characteristics. It is believed that the deterioration of the fluorescent characteristics caused by the temperature increase is affected by the bonding strength of compounds and a size difference between the activators and the host lattice. Particularly, since conventional white light emitting diode-based products such as automotive head light, indirect illumination, etc. have operating temperatures up to 150° C., there is a need of developing phosphors and light emitting diodes that experience little variation in optical characteristics such as light intensity and chromaticity coordinate, and exhibit a superior stability even at high temperatures.
As mentioned above, the light emitting diode can realize white light using such phosphors that function as frequency converting means. Specifically, with the phosphors disposed above the light emitting diode chip, the light emitting diode obtains the white light through color mixing of some parts of primary light emitted from the light emitting diode chip and secondary light, of which frequency is converted by the phosphors. Since such a white light emitting diode is cheap and operated by very simple principles and configurations, it is widely employed in the art.
For instance, for a white light emitting diode wherein phosphors for emitting yellow-green or yellow light based on blue light emitted from a blue light emitting diode chip as an excitation source are applied to the surface of the blue light emitting diode chip, it is possible to obtain white light through combination of the blue light emitted from the light emitting diode chip and the yellow-green or yellow light from the phosphors. However, such a white light emitting diode exhibits low color rendering due to lack of a spectrum in green and red regions relating to the phosphors emitting single yellow light, and, in particular, the white light emitting diode is difficult to realize natural or similar colors due to a low color purity after transmission of light through a color filter, when employed as a light source of an LCD backlight unit.
In order to solve the problems as described above, another conventional white light emitting diode includes a blue light emitting diode chip and phosphors capable of being excited by blue light emitted from the blue light emitting diode chip and emitting green light and red light. With this configuration, it is possible to realize white light having a high color rendering of 85 or more by mixing the green light and the red light emitted from the phosphor excited by the blue light. Since the white light emitting diode has a very high conformity to a color filter, a component of LCD, when employed as the light source of the LCD backlight unit, it has a merit in that the white light emitting diode can realize images closer to natural colors with its high color purity after transmission of light through the color filter.
Representative examples of the green light emitting phosphors include an orthosilicate phosphors and a thiogallate phosphor, both of which exhibit excellent blue light-based excitation efficiency. Here, the thiogallate phosphor is a sulfide-based phosphor expressed by (Ca, Sr, Ba)(Al, Ga, In)2S4:Eu, and has little influence on adjacent spectrums due to its very narrow full width at half maximum of 50˜60 nm in a light emitting spectrum as well as the blue light-based excitation excellent efficiency, realizing very high color reproducibility when applied to the light source of the ICD backlight unit. However, the thiogallate phosphor has a problem in that it is likely to react with moisture, causing variation in chemical characteristics of the phosphor.
Further, representative examples of the red light emitting phosphors include sulfide-based phosphors, such as (Ca, Sr)S:Eu, (Zn, Cd)(S, Se):Ag, etc., and nitride-based phosphors, such as (Ca, Sr, Ba)2Si5N8:Eu, CaAlSiN3:Eu, Ce(Ca, Sr, Ba)Si7N10:Eu, CaSiN2:Eu, etc., which have been recently developed. For the nitride-based phosphors, although it is possible to achieve an excellent chemical stability, the full width at half maximum of the light emitting spectrum exists in a very wide range of 90˜110 nm and overlaps with an adjacent green light spectrum, providing a relatively low color purity after transmission of light through the color filter when the nitride-based phosphors are employed as the light source of the LCD backlight unit.
Further, since the sulfide-based phosphors enable adjustment of a frequency in the range of 600˜660 nm depending on a composition thereof and has a very narrow full width at half maximum of 60˜70 nm in the light emitting spectrum, they can realize higher color reproducibility when employed as the light source of the LCD backlight unit. However, the sulfide-based phosphors have problems in that they are likely to react with moisture, carbon dioxide, etc. in atmosphere and become oxides or carbonate, causing variation of the chemical characteristics of the phosphors. In addition, H2S gas generated by a chemical reaction between the sulfide-based phosphors and the moisture changes the fluorescent characteristics of the phosphors to cause a rapid reduction of luminous intensity along with variation of the chromaticity coordinate. Furthermore, the H2S gas corrodes electrodes formed of metal such as Ag or Au, deteriorating reliability of the light emitting diode.
Particularly, the sulfide-based phosphors, such as (Ca, Sr)S:Eu, SrGa2S4:Eu, ZnS:Cu, Al, or (Zn, Cd)(S, Se):Ag, are likely to react with the moisture and lose their inherent fluorescent characteristics, causing a significant reduction of the luminous intensity and variation of the optical characteristics. As a result, the oxide-based phosphors and the sulfide-based phosphors are limited in their applications.